1. Field of the Invention
This invention relates to ion scattering spectrometers in which the surface of a target to be analyzed is bombarded with a beam of monoenergetic ions and the mass of atoms on the surface is determined from a measurement of the loss of energy of ions thereupon scattered from the surface.
2. Description of the Prior Art
U.S. Pat. No. 3,480,774 (Smith) assigned to the assignee of the present invention and from which the present invention is a direct descendant, describes the original basis for what has now become an important commercial technique of surface analysis, i.e., ion scattering spectroscopy (ISS). Since the issuance of that initial patent, in which a large, cumbersome and expensive apparatus was described, numerous improvements have been made in which the size and expense of ISS systems has been decreased, while nevertheless providing gains in sensitivity, resolution, convenience and broader applicability. See, for example, U.S. Pat. Nos. 3,665,182 (Goff and Smith), 3,665,185 (Goff), 3,916,190 (Valentine and Goff), 3,916,191 (Leys and Goff) and 3,920,989 (Erickson and Smith), all of which patents are assigned to the same assignee as the present application.
Similarly, the increasing commercial acceptance of ISS has spurred the activities of other investigators and has thereby promoted the progress of this technology. For example, U.S. Pat. No. 3,920,990 (Nieuwland et al) depicts an ISS device in which a cylindrical mirror analyzer is provided with an annular entrance diaphragm. Such a configuration is said to enable the acceptance into the analyzer of as many as 180 times as many ions as were accepted into the analyzer described in U.S. Pat. No. 3,480,774 (Smith).
In that device, the ion gun is axially positioned external to the analyzer such that the incident ion beam passes axially through the entire analyzer. Scattered ions are then detected by a concentric annular detector which is presumably shielded from the incident ion beam but appears not to be positioned for minimum trace width and hence may significantly degrade the performance of the analyzer. The long ion trajectories and attendent problems associated with focusing and with space charge spreading make it difficult to reduce the diameter of the resultant beam. Similarly, magnetic filters, such as the Wien filter there disclosed must be shielded from the analyzer to avoid distortion of ion trajectories therewith.
In order to permit surface analysis of a smaller size with ISS systems, further attention has been directed to the development of improved ion sources. One such source is disclosed in U.S. Pat. No. 3,937,958 (Rusch and Sievers), which patent is also assigned to the assignee of the present invention, and is said to be usable in an ISS such as that disclosed in U.S. Pat. No. 3,665,182 (Goff and Smith).